This invention relates to telecommunications systems for, and methods of, transferring telecommunications information through telephone lines. More particularly, this invention relates to systems for, and methods of, transferring streaming data such as digitally encoded video and voice signals efficiently and reliably through telephone lines.
Telephone systems in the United States provide central office for receiving signals from calling telephones within a particular radius such as one (1) to two (2) miles from such central office. The telephone signals from the calling telephone are then transmitted through long distances from such central office. The telephone signals then pass to the receiving telephone through a second central office within a radius of one (1) mile to two (2) miles from a receiving telephone. The telephone signals then pass from the second central office to the receiving telephone.
The telephone signals are transmitted long distance between central offices through optical fibers which have replaced other media previously provided for such purposes. The optical fibers have certain distinctive advantages over the lines previously provided. They allow a significantly increased number of signals from different telephones to be transmitted at the same time through the optical fibers. They pass the digitally-encoded signals with a higher accuracy than other media.
Various systems have been adopted to carry digitally-encoded signals for telephone, video, and data services. One of such systems now being adopted is designated as asynchronous transfer mode (ATM). This system is advantageous because it recognizes that generally signals travel in only one direction at any one time between a calling subscriber and a receiving subscriber. The system preserves bandwidth in the other direction so that a maximum number of different messages can be transmitted in such other direction.
In ATM systems, cells are provided to transmit information between access multiplexers or terminals through central offices. Each of the cells contains headers identifying the calling and receiving stations and also contains a payload providing the information being transmitted and received. The cells pass from the calling telephone through the access multiplex to a first central station. The cells then pass through the first central station and optical fibers to a second central station and then to the receiving access multiplex. During the transfer of the cells to the central stations, the headers may be changed. These changes in the address indicate the path that the cell is following between each pair of central stations to reach the receiving telephone.
In the prior art, to reassemble cells into signals at the access multiplex, the header and the payload in each cell have been transferred to a control memory that processes the header to determine what path it came from so that the signal can be reassembled based upon this path. This has created certain difficulties. For example, it has required the control memory to be relatively large, particularly since the memory receives the header and the payload. It has also caused the transfer to be slow, particularly since the header and the payload have to be processed and the payload is generally twelve times longer than the header.
U.S. Pat. No. 5,949,781 discloses a system for, and a method of, overcoming the disadvantages discussed in the previous paragraphs as occurring in an ATM system. The system and method disclosed and claimed in U.S. Pat. No. 5,949,781 minimize the time for processing the cells to update the headers as the cells are transferred through the telephone lines between the calling telephone and the receiving telephone.
In one embodiment of the invention disclosed and claimed in U.S. Pat. No. 5,949,781, a header and a payload in a cell are separated for transfer between a cell interface and a host memory. The header is transferred to a control memory. The header is transferred to a control memory. For transfer to the host memory, the control memory initially provides a host-memory region address and the region length. The payload is recorded in such region address. The control memory also provides a second host-memory region address, and length, when the payload length exceeds the payload length in the first region address. For transfer from the host memory to the cell interface, the control memory provides a host memory region address. The cell interface passes a payload from such region address.
The system disclosed and claimed in U.S. Pat. No. 5,949,781 processes data which is segmented into packets and in which the packet data is further segmented into cells. As discussed above, the cells are then transferred into region addresses in a host memory buffer. The system disclosed and claimed in U.S. Pat. No. 5,949,781 relates primarily to packet data such as is provided for legacy LAN traffic.
In addition to packet data, ATM systems deal with streaming data such as voice and video. This data occurs at a regular rate and is not necessarily segmented into relatively large data units such as packets. Furthermore, the final end point for streaming data may not be the host CPU. Instead, it may be a special audio or video subsystem.
Providing a segmentation and reassembly (SAR) system oriented toward the use of a host memory buffer for streaming data has several disadvantages including the following:
1. The streaming data must be copied into the host memory buffer.
2. Buffer management and communication of buffer information between the host and the SAR system requires computing resources from the host computer.
3. In order to keep the overhead of the host memory buffer to a minimum, the buffer must have a size of several ATM cells. This adds to the latency of the streaming data.
4. Additional memory must be provided for the host buffers.
This invention provides a system in which the packet data is transmitted and received as discussed above. In addition, the invention provides a system for, and method of, transmitting and receiving the streaming data. In this invention, the streaming data is transmitted from, and is received, in FIFO""s. This minimizes the complexity and cost of the system.
In one embodiment of the invention, data (e.g. legacy LAN traffic) segmented into packets provide a header and a cell payload for each cell in each packet. The cell payloads are transferred to a region address in a host memory in accordance with a determination by a control memory. When the cell payload is to be transmitted from the host memory, the cell payload for a particular region address is combined with the header stored in the control memory for such address.
Streaming data (e.g. voice or video) occurs at a regular rate and is not necessarily broken into packets. Furthermore, the end point for the data may not be the host CPU. In this invention, the streaming data is segmented to provide cell headers and cell payloads. The cell payloads are then transferred to a host receive FIFO in accordance with a determination by the control memory and are stored in a data sink. Cell payloads from a data source are transferred into a host transmit FIFO at a particular rate and are transferred from the host transmit FIFO preferably at a substantially constant rate higher than the particular rate. Such cell payloads are combined with the headers in the control memory.
When the cell payloads in the host transmit FIFO become almost depleted, the transmission of the cell payloads from such other FIFO is skipped to allow such other FIFO to receive additional cell payloads from the data source. The transmit and receive FIFO""s respectively maintain cell alignment by providing particular controls with respect to the minimum and maximum number of cell payloads in the FIFO""s.